(a) Field of the Invention
The present invention relates generally to a semiconductor optical device having a strained quantum well structure and, more particularly, to a semiconductor optical device including a strained quantum well structure having superior crystal interfaces and excellent crystallinity.
(b) Description of the Related Art
A semiconductor optical device having a strained quantum well structure, in which a plurality of III-V compound semiconductor active layers having different compositions are laminated on a III-V compound semiconductor substrate, is widely used as a semiconductor light emitting device, a semiconductor optical amplifier, a semiconductor light receiving device or the like.
In the strained quantum well structure, compressive and tensile strains are applied to the active layers of the quantum well structure. That is, a quantum well layer and a barrier layer, which constitute the quantum well structure as combined, are formed as a compressive strained layer and a tensile strained layer, respectively, or vice versa. The strained quantum well structure has been proposed in order to provide a semiconductor optical device having a lower threshold and improved characteristics to enhance the performance and functions of optical communication devices and optical communication networks.
A semiconductor optical device having a conventional strained quantum well structure will be described first. Referring to FIG. 1, a semiconductor optical device 80 having a conventional strained quantum well structure includes an n-type InP buffer layer 84, a multiple-quantum well (hereinafter referred to as an "MQW") structure 86, a p-type InP cladding layer 88 and a p-type GaInAs contact layer 90, which are laminated in this order on an n-type InP substrate 82. The MQW 86 has a strained structure in which a combination of a 20 nm-thick undoped In.sub.1-X Ga.sub.X P (X=0.08) layer 92 and a 10 nm-thick undoped InAs.sub.Y P.sub.1-Y (Y=0.45) layer 94 is laminated for five times, for example, in a cyclic order.
In the MQW 86 of the semiconductor optical device 80, each InAs.sub.Y P.sub.Y-1 (Y=0.45) layer 94 forming the quantum well layer has a compressive strain of 1.45% acting parallel to the main surface of the substrate while each In.sub.1-X Ga.sub.X P (X=0.08) layer 92 forming the barrier layer has a tensile strain of 0.56% acting parallel to the main surface of the substrate, whereby the strained quantum well structure is obtained in which the strains of the compound semiconductor crystal layers having different compositions are counterbalanced. Contrary to the above example, the strained quantum well structure may also be constructed such that each quantum well layer forms a tensile-strained layer while each barrier layer forms a compressive-strained layer.
Due to the strain compensation in the layer combinations, the net strain is theoretically reduced down to 0.11% in the quantum well structure, so that a strained multiple-quantum well structure can be obtained in which the layer combinations each including the quantum well layer and the barrier layer can be theoretically laminated for as high as about 50 times without degradation.
However, the conventional strained quantum well structure as described above will not actually exhibit superior crystallinity and laser characteristics. For example, if a sample semiconductor optical device having the conventional strained quantum well structure is actually manufactured and the crystal planes of the compressive-strained layer and the tensile-strained layer of the strained quantum well structure are inspected, it will be revealed that the crystal planes are rough and hence a superior crystal interface is not formed between the compressive-strained layer and the tensile-strained layer. In addition, if photoluminescence is measured for evaluation of the crystallinity, it will be also revealed that the half-width of the spectrum is almost double the theoretical value of an ideal strained quantum well structure.
In short, it is difficult to obtain excellent laser characteristics in an actual product including a strained quantum well structure.
In the present specification, the term "crystallinity" means the regularity of the arrangement of atoms forming a crystal structure. Therefore, if the amount of crystal defects in the crystal structure are insignificant for device characteristics, it can be recited that the crystal has a superior or excellent crystallinity.